[ticket/11906] Add missing configs from notifications cron to schema_data.sql

PHPBB3-11906
[ticket/11906] Add notification purge cron to cron tasks

PHPBB3-11906
[ticket/11906] Fix namespace issues in prune_notifications

PHPBB3-11906
Merge remote-tracking branch 'nickvergessen/ticket/11906' into develop

# By Joas Schilling
# Via Joas Schilling
* nickvergessen/ticket/11906:
  [ticket/11906] Fix namespace issues in prune_notifications
  [ticket/11906] Add notification purge cron to cron tasks
  [ticket/11906] Add missing configs from notifications cron to schema_data.sql